1. Field of the Invention
The present invention relates to an optical disc having grooves and pits with different depths, and an easy method of manufacturing a disc having grooves and pits with different depths using a stamper obtained by stamping from a master an even number of times.
2. Description of the Related Art
Rewritable, erasable and reproducible optical discs, such as digital versatile disc random access memory (DVD-RAM) and DVD-rewritable (RW), include grooves for recording and/or reproduction. The main difference between DVD-RAM and DVD-RW is in their recording regions. In particular, information can be written to both land and groove regions of a DVD-RAM, and the DVD-RAM has separated addresses formed as pits, such that desired information can be searched throughout the DVD-RAM by such physical units. In contrast, as for the DVD-RW, information can be written to only the groove region, and block addresses, instead of the pits of the DVD-RAM, are formed in the land region (i.e., land pre-pits). However, pits can also be formed in the land region of the DVD-RW when reproduction-only information, i.e., data for copy protection, is required.
The width and depth of the grooves in a DVD-RW are determined for excellent jitter characteristics without pits. An appropriate depth of the DVD-RW grooves is about 20-40 nm, which can be expressed as λ/12n, using the wavelength (λ) of a laser beam and the refractive index (n) of the disc. This groove depth of the DVD-RW is smaller than that of DVD-RAM, which is expressed as λ/6n. In case of the DVD-RW, if pits are formed to have the same depth as that of the grooves at λ/12n, the following problems occur.
As shown in FIG. 1, the pit depth for the DVD-RW can be determined based on the amplitude ratio of a reproduction signal. Referring to FIG. 1, the amplitude of a reproduction signal from pits was measured with respect to variations of pit depth in units of λ at a wavelength of 650 nm, a numerical aperture (NA) of 0.6, and a minimum mark length of 0.42 μm, and then normalized with respect to a maximum amplitude of a reproduction signal, i.e., at the groove depth of a DVD-RW. The refractive index (n) of the disc is ⅕. When a record mark length is 3T and 14T, the amplitude ratio of each reproduction signal is m1 and m2, which ranges between 0.2 and 0.3, at a pit depth of λ/12n, i.e., about 0.056 λ. For a pit depth of λ/4n, i.e. about 0.167 λ, the amplitude ratio of the reproduction signal is close to 1 at the point s.
As shown in FIG. 1, when the pit depth is equal to the groove depth at λ/12n, the signal level is 30% or less of the signal level at the pit depth of λ/4n. Such a pit signal level from the same pit and groove depth is unreliable. Therefore, a new disc having grooves and pits of different depths would improve pit reproduction signal levels.
A conventional method of manufacturing a disc having grooves and pits having different depths is illustrated in FIGS. 2A through 2H. As shown in FIG. 2A, a photoresist 103 is deposited over a glass master 100 and then exposed to cutting by laser beams having different powers Pw1 and Pw2. After a developing process, a pit region 104 and a groove region 105 having different depths h1 and h2 are obtained, as shown in FIG. 2B. Following this, the glass master 100 with the pit region 104 is etched using CF4 gas (first etching step) such that the pit region 104 become deeper as shown in FIG. 2C.
As shown in FIG. 2D, the groove region 105 is exposed by ashing using O3 gas, and then subjected to etching using CF4 gas (second etching step). As a result, the pit region 104 and the groove regions 105 having different depths are formed, as shown in FIG. 2E. Then, as shown in FIG. 2F, the remaining photoresist 103 is removed by ashing (second ashing) using O3 gas, thereby resulting in a master 106 having the pit region 104 with a depth s1 and the groove region 105 with a depth s2, wherein the depth s1 is larger than the depth s2.
A thin film is deposited over the master 106 with the pit region 104 and the groove region 105 by sputtering, as shown in FIG. 2G. Then, a stamper 110 is stamped from the master 106, wherein the stamper 110 has an inverse shape to that of the master 106, as shown in FIG. 2H. The stamper 110 is used to manufacture a substrate having pits and grooves with different depths by injection molding. Here, the bumps and recessions of the master 106 are printed as recessions and bumps in the stamper 110, and the substrate molded from the stamper 110 has the same shape as the master 106.
According to the conventional disc manufacturing process, the etching is performed twice, so that the entire process is complicated with low yield. Further, because the conventional disc manufacturing process uses only one stamper, the photoresist should be etched. In addition, the manufacturing cost is high.